In recent years, the degree of integration of semiconductor integrated circuits and other microelectronic devices has continued to increase, with concomitant increases in the intricacy and complexity of their circuit patterns. Increased complexity typically means more layers, larger layers, and smaller circuit elements that must be formed in each layer. Hence, overlay of successive layers relative to previously applied layers must be performed with correspondingly greater accuracy.
Conventionally, optical steppers are employed mainly for performing microlithography aspects of wafer processing in the manufacture of semiconductor integrated circuits. In optical microlithography, the reticle normally is produced by direct-writing using an electron beam. An advantage of optical microlithography is the ability to expose an entire reticle pattern in one exposure (“shot”), which typically provides excellent “throughput” (number of wafers that can be exposed with a pattern per unit time).
Unfortunately, optical microlithography currently is limited by the diffraction of light from providing ever-increasing pattern resolution. This has provided the impetus to find and develop alternative microlithography techniques that can provide higher resolution. An attractive candidate alternative technique is microlithography performed using a charged particle beam such as an electron beam or ion beam. Charged-particle-beam (CPB) microlithography offers prospects of better resolution for reasons similar to the substantially improved resolution obtained with electron microscopy compared to optical microscopy.
A disadvantage of CPB microlithography is lower throughput than normally obtained with optical microlithography. Various approaches have been investigated to find a practical CPB microlithography technique with acceptable throughput. Certain techniques that have received attention include the “partial-pattern-block” exposure techniques, including “cell projection,” “character projection,” and “block exposure.” Partial-pattern-block exposure techniques are employed especially for patterns in which a basic unit, such as a memory cell, is repeated a large number of times. For example, for a DRAM, the basic unit can have dimensions of approximately 5-μm square on the substrate. The reticle usually defines multiple basic units that are transfer-exposed repeatedly and separately using an electron beam. Not only is substantial time required to form an entire pattern on the wafer in this manner, but also portions of the pattern that are not highly repeated typically must be exposed using another technique such as “variable-shaped beam” direct writing. Consequently, throughput obtained with these techniques is too low for large-scale mass-production of integrated circuits.
An approach offering prospects of substantially improved throughput compared to partial pattern block exposure involves exposure of an entire reticle pattern (or even multiple reticles) in one “shot,” similar to optical microlithography. The general idea is to expose the reticle with “reduction” (demagnification), meaning that the pattern image as formed on the substrate is smaller than the pattern as defined on the reticle. Unfortunately, this approach has not been realized from a practical standpoint. First, it has been impossible to date to produce a reticle that can be exposed in a single shot of a charged particle beam. Second, this approach requires an extremely large CPB-optical system. It has been impossible to date, using such large CPB lenses and deflectors, to achieve satisfactory correction of aberrations, especially off-axis aberrations.
In view of the problems inherent in the approaches summarized above, yet another approach has been the subject of extensive investigation. This approach is termed the “divided-reticle” approach, in which the reticle pattern is divided into multiple exposure units typically called “subfields” each defining a respective portion of the overall pattern. The subfields are exposed individually and in a particular exposure order, with demagnification. The subfield images are formed on the substrate in locations that result in all the images being “stitched” together properly and in a contiguous manner so as to form the entire pattern. Even though the throughput obtained with the divided-reticle approach is not as high as with the full-reticle exposure technique, the throughput is substantially better than obtained with the partial-pattern-block exposure technique. Furthermore, each subfield can be imaged with high accuracy and with excellent correction of aberration and of errors of magnification and image position on the substrate. On the reticle, the subfields typically are arranged in a rectilinear array of rows and columns of subfields that are exposed subfield-by-subfield in each row and row-by-row. As each subfield in a row is exposed, the charged particle beam is deflected in a lateral direction as required, within the optical field of the CPB-optical system. Progressing from one row to the next is accomplished by appropriate movements of the reticle and substrate.
Reticles usable in CPB microlithography typically are of two types: stencil reticles and membrane reticles. A stencil reticle comprises a relatively thick CPB-scattering membrane, wherein the pattern elements are defined by a corresponding pattern of through-holes in the membrane. Whereas charged particles of an incident “illumination beam” are scattered as they pass through the membrane, charged particles of the illumination beam incident on a through-hole pass with little to no scattering through the through-hole. A membrane reticle comprises a relatively thin CPB-transmissive membrane, wherein the pattern elements are defined by a correspondingly patterned scattering layer formed on the CPB-transmissive membrane. Whereas charged particles of an illumination beam incident on a region of the reticle lacking any of the scattering layer pass through the membrane with little to no scattering, charged particles are highly scattered if incident on a region of the scattering layer.
One difficulty with a stencil reticle is the so-called “donut problem,” characterized by a pattern element that must be defined by a through-hole surrounding a portion of the reticle membrane. The problem is that a stencil reticle provides no way in which to provide physical support for the surrounded portion of the reticle membrane. To achieve exposure of a “donut” pattern element, either two separate portions of the same reticle, or two separate reticles, must be used, each defining a portion of the “donut” while providing support for the surrounded portion of the membrane. The two reticle portions needed to achieve full exposure of the “donut” element are termed “complementary.” This scheme is depicted in FIGS. 21(A)-21 (B). In FIG. 21(A), a “donut” pattern element 101 is shown. To achieve full exposure of the donut pattern element 101, the element is divided into a first portion 103 and a second portion 104 that must be exposed separately. Hence, two exposures are required to expose the entire donut element 101 onto the substrate.
More specifically, FIG. 21(A) shows the donut pattern element 101 needing to be defined by a respective through-hole on a stencil reticle. If defined on a single region of a reticle, this would result in a ring-shaped through-hole 102R surrounding an unsupported island region 102C that is not exposed on the reticle. The donut pattern element 101 simply cannot be defined on a single region of a stencil reticle because the region of the stencil reticle provides no way in which to support the island portion 102 surrounded by the ring-shaped through-hole 102R. Hence, the donut pattern element 101 is divided along the line 100 into laterally symmetrical complementary elements 103, 104 that are defined by different subfields on the same reticle or on separate reticles. Transfer of the complete donut pattern element 101 requires two exposures. Also, exposure must be performed with sufficient accuracy ensuring that the images of the two portions 103, 104 are “stitched” together properly on the substrate.
FIG. 21(B) depicts use of a complementary reticle to help define a long linear pattern element 105. The pattern element 105 is divided into segments 108, 110 that are defined on a first region of the reticle (or on a first reticle) and a segment 109 that is defined on a second region of the reticle (or on a separate, second reticle). This manner of division typically is used for extremely long linear pattern elements. Whereas the linear element 105 logically could be formed on a single stencil reticle, long pattern elements 105 are especially vulnerable to splitting or other instability of the reticle. To prevent such problems, long linear elements 105 typically are divided (along lines 106, 107) into complementary portions 108, 109, 110.
Real-life technical requirements for microlithography in wafer-fabrication plants include the following:                (1)-higher pattern-transfer accuracy to achieve the target level of device-pattern miniaturization;        (2) higher throughput to handle mass-production of devices; and        (3) increased chip size accompanying progress in semiconductor-device complexity.For example, as chip-size increases, it is not always possible to form all the pattern elements, especially elements subject to complementary patterning as summarized above, on a single reticle. This results in an increase in the number of reticles that must be used. This increases the time required to perform an exposure of an entire pattern, with a corresponding drop in throughput.        